Transformers using mineral oil as a dielectric and coolant generally employ windings in which cooling channels are maintained in the windings by the use of ribs to space the winding layers apart. The coolant is caused to flow by natural convection through the channels to cool the winding. The fluid is cooled by an external heat exchanger and the cool fluid is returned to the bottom of the channel. The heat transfer within the winding is determined by the flow rate and boundary layer thickness and is influenced predominantly by the fluid density and viscosity. The channel thickness ranges from 0.188 minimum to about 0.50 inch. The channel width is determined by the short circuit forces which must be resisted by the coil conductor and ranges from about 1 inch to 3 inches. Transformers are also constructed utilizing air or gas as a coolant with duct thickness of 0.375 minimum with 0.50 inch normally used. The natural convection heat transfer is also dependent upon any obstructions in the coolant channel which might occur in the winding manufacture. Because of these obstructions such as layer insulation, crooked spacers, etc., duct sizes below the minimum determined from experience are not used. Since the state of the art of providing cooling ducts for windings for mineral oil transformers is well established manufacturers have also utilized the same geometries for vaporization cooled transformers. FIG. 1 shows a prior art winding described within U.S. patent application Ser. No. 843,676 entitled "Percolation Cooled Electrical Transformers". A rectangular winding 10 is provided with rectangular cooling duct 11 approximately 3/16 inch wide by 11/2 inches long, formed by inserting rectangular spacers 12 into the winding during manufacture.
FIGS. 2 and 3 show the prior art method of manufacturing the cooling channel spacers 12. Rectangular strips of pressboard 12' are glued to a thin sheet of paper 13 which is inserted between the winding layers during winding. On round windings the paper strip 13 may be omitted and the strips 12' are inserted into the winding during the coil winding process and held in place by the pressure exerted by the wire due to the tension applied during the winding. The strips 12' may also be made of round fiber rods or other shapes or materials to maintain the coolant duct size. Some manufacturers use a cooling duct 11 formed by means of a thick corrugated paper board 14 as shown in FIG. 4.
The use of coolant ducts 11 for example as described in the prior art is undesirable for vaporization cooled transformer windings because the method increases the radial dimensions ("build") of the winding and the length of the wire required. The additional length of wire increases the winding resistance and resulting electrical losses which may be dissipated by the condenser. The condenser size must therefore be larger than if a smaller winding with lower losses could be used. The losses may be reduced by increasing the conductor cross-section area, but this results in an increased quantity of conductor material and core steel. In addition, a smaller radial build further reduces the tank size and the quantity of fluid required. There is also labor involved in the manufacture of the cooling ducts and their insertion into the winding. The paper sheets to which the strips are attached increases the temperature of the winding which must be compensated for by increases in the conductor or condenser sizes. With a decreased radial build of the winding, therefore, less core steel, conductor material, tank, liquid and condenser surface area are required so that substantial reductions in transformer weight and cost can be realized.
It is an object of this invention to reduce the radial build of vaporization cooled transformer windings by means of a novel method of integrally forming reduced size coolant ducts without the use of rectangular strips, ribs and corrugated boards as commonly used in transformer manufacture.